1. Field
Embodiments relate to a walking control apparatus of a robot which stably walks using a plurality of legs, and a method of controlling the same.
2. Description of the Related Art
A robot is a machine which has a joint structure similar to that of a human and performs the same operations as the hands and feet of a human using the joint structure.
Initially, industrial robots for automated and unmanned production tasks were developed. However, recently, a service robot to provide various services to a human has been actively developed.
Such a service robot mostly provides a service to a human while walking similar to a human. Accordingly, research into walking of the robot has been actively conducted.
Examples of a walking control method of a robot include a position-based Zero Moment Point (ZMP) control method (which follows a desired position of a robot joint), a torque-based dynamic walking control method (which follows desired torque of a robot joint), and a Finite State Machine (FSM) control method.
In the ZMP control method, a walking direction, a stride width, a walking rate and the like are determined in advance to satisfy a ZMP constraint, that is, a condition in which a ZMP is present in a safe area (which corresponds to the area of one foot in the case where the robot is supported by one foot or corresponds to a small area which is set in consideration of safety in a convex polygon including the areas of two feet in the case where the robot is supported by two feet) of a stance polygon formed by stances of legs of the robot, the walking pattern of each leg corresponding to the determination is generated, and the walking trajectory of each leg is calculated according to the walking pattern.
The position of the joint of each leg is calculated by inverse Kinematic calculation of the calculated walking trajectory, and a desired control value of each joint is calculated based on the current angle and the desired angle of each joint.
The torque-based dynamic walking control method is implemented by servo control to enable each leg to follow the calculated walking trajectory during every control time period. That is, it is detected whether the position of each leg accurately follows the walking trajectory according to the walking pattern while walking. When each leg deviates from the walking trajectory, the torque of the motor is controlled such that each leg accurately follows the walking trajectory.
In the FSM control method, the robot does not walk to follow the position during every control time period, operation states of the walking robot are set in advance, desired torques of joints are calculated by referring to the operation states (indicating the states of the FSM) while walking, and the robot walks to follow the desired torques of the joints.
In the FSM control method, the robot adopts various poses by changing the operation state while walking. However, since each pose is adopted in a restricted operation state, a separate operation to maintain balance of the robot is performed regardless of a walking operation to perform a task.
Since the ZMP control method is the position-based control method, accurate position control is possible, but high servo gain is necessary because accurate angle control of each joint is performed. Accordingly, since high current is necessary, energy efficiency is low and joint rigidity is high, thereby applying considerable shock to walking surfaces.
In order to calculate the angle of each joint from the walking pattern of the foot and a given Center Of Gravity (COG) through inverse kinematics, Kinematic Singularity needs to be avoided. Thus, the robot always bends its knees while walking. Thus, the robot may unnaturally walk unlike a human.
In the torque-based dynamic walking control method, a dynamic equation needs to be solved for stable walking. However, since the dynamic equation of a robot having legs with six degrees of freedom to implement a certain direction in a space is very complicated, such a method has been applied to a robot having legs with four degrees of freedom.
In the FSM control method, since control is performed by a torque command and an elasticity mechanism is applied, energy efficiency is high and rigidity is low, thereby providing safety to surroundings. However, since it is difficult to perform accurate position control, it is difficult to perform accurate whole-body motion such as ascending of stairs or avoidance of an obstacle.